Technical Field
The present disclosure relates to a package for semiconductor devices sensitive to mechanical and thermo-mechanical stresses, such as MEMS pressure sensors.
Description of the Related Art
As is known, semiconductor devices, such as integrated circuits and MEMS devices, are generally encapsulated in packages for their protection and handling. Hereinafter, reference is made to packages that allow surface mounting.
Presently, the most used type of surface mounted packages for MEMS sensors is the so-called LGA (LAND grid array) package, which has a square or rectangular grid of contacts on the underside of the package.
However, the standard LGA package for MEMS pressure sensors is not compliant with automotive requirements where a solder inspection of the printed circuit board is provided for. Therefore, so-called quad-flat no-leads-single row (QFN-Sr), also called micro leadframe (MLF) and SON (small-outline no leads), packages are used for automotive applications, that is packages that allow surface mounting, and have one peripheral row of terminals on the bottom surface.
In standard QFN technology, the leadframe is post-molded, and the empty spaces are filled using resins having a coefficient of thermal expansion (CTE) close to that of the metal leadframe, in order to avoid delamination. However, these resins have high elastic modulus and the package is quite stiff. As a consequence, mechanical and thermo-mechanical stresses are transmitted to the pressure sensor after soldering the package on PCB card, resulting in offsets and drifts. The reading of the sensor is thus negatively affected by noise due to stresses generated on the vehicle and transmitted to the sensor by the stiff package; or by stresses generated during the surface mounting process, due to solder connections cooling; by stresses generated during mounting on the vehicle, due to mechanical tolerance mismatch, or by stresses generated during the device life, due to cockpit temperature changes.
In different packages, so called premolded QFN, it is possible to have a leadframe with a pre-molded plastic cavity. This solution solves the problem of avoiding stress transmission by the resin of the package but not by stresses transmitted by the copper leadframe.
Therefore, standard QFN packages pre- or post-molded cannot be used for MEMS pressure sensors intended for automotive application.
On the other hand, the use of low-modulus substrate LGA is not possible due to the risk of substrate warpage and poor ultrasonic transmission during ultrasonic wire bonding.